This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A073312 #17 Dec 07 2023 01:44:31
%S A073312 0,0,0,0,1,0,1,0,2,1,3,0,4,1,2,1,5,0,6,1,4,1,7,0,7,2,5,3,11,0,11,3,7,
%T A073312 3,9,1,13,3,7,2,14,1,14,3,6,4,16,1,16,3,11,5,20,2,15,4,13,5,22,1,23,5,
%U A073312 10,6,18,2,25,6,15,2,26,2,27,6,11,7,24,2,29,4,17,8,31,1,23,8,17,8,33,1,28
%N A073312 Number of nonsquarefree numbers in the reduced residue system of n.
%H A073312 Amiram Eldar, <a href="/A073312/b073312.txt">Table of n, a(n) for n = 1..10000</a>
%F A073312 a(n) + A073311(n) = A000010(n).
%F A073312 Sum_{k=1..n} a(k) ~ c * n^2, where c = (3/Pi^2) * (1 - A065463) = 0.0898387... . - _Amiram Eldar_, Dec 07 2023
%e A073312 n=15, there are A000010(15)=8 residues: 1, 2, 4=2^2, 7, 8=2^3, 11, 13 and 14; two of them are not squarefree: 4 and 8, therefore a(15)=2.
%t A073312 a[n_] := EulerPhi[n] - Module[{rad = Times @@ (First@# & /@ FactorInteger[n])}, Sum[MoebiusMu[k*rad]^2, {k, 1, n}]]; Array[a, 100] (* _Amiram Eldar_, Mar 08 2020 *)
%Y A073312 Cf. A073311, A013929, A000010, A048864, A048865.
%Y A073312 Cf. A065463, A104141.
%K A073312 nonn
%O A073312 1,9
%A A073312 _Reinhard Zumkeller_, Jul 25 2002